Analytical instruments which detect one or more characteristics of a fluid are commonly employed in a wide variety of applications, such as sample purification, chemical analysis, clinical assay, and industrial processing. Gas and liquid chromatographs are particular examples of analytical instruments wherein certain characteristics related to a particular fluid are detected, e.g., the presence or absence of a fluid component, such as an analyte or contaminant.
Capillary chromatographic separation methods are preferably performed in synthetic fused silica tubing with internal diameters ranging from 5 to 530 micrometers. Such tubing consists of a silica (SiO.sub.2) glass drawn at high temperature from a quartz preform provided with a protective outside layer such as polyamide or aluminum. See, for example, U.S. Pat. No. 4,293,415, issued to Dandeneau et al., which disclosed the use of a fused silica capillary having wall coatings on the inside surface to stimulate specific interactions when employed in open tubular capillary gas or liquid chromatography, for open tubular supercritical fluid chromatography. Fused silica capillary columns are also known for use in capillary electrophoresis and capillary electro-chromatography.
The resolution of a capillary column, and the time required for carrying out a separation, are functions of several interrelated column and operational parameters. The major factors that influence a separation are: column internal diameter, column length, the type of stationary phase and its film thickness, the type of carrier gas, the carrier gas velocity, and the column temperature. Among these factors, a reduction in the column length is understood to decrease the analysis time, but at the expense of resolution. For fast capillary gas chromatography without a significant loss in resolution, the column internal diameter is typically reduced to 0.1 mm or less. Hence, there are advantages and limitations in the use of narrow-bore well coated open tubular column (WCOT) for high-speed capillary gas chromatography. In a typical example for use in current instrumentation, a 100 micrometer internal diameter column of 5 to 10 meters in length may be used.
However, the most serious limitation of the reduced diameter column is its associated decrease in sample capacity. Capacity is known as the ability of a column to tolerate high concentrations of solutes. Degradation of chromatographic performance is observed when the column capacity is exceeded. This condition is commonly referred to as "overload" and is indicated by peak broadening and asymmetry. Sample capacity is also related to the film thickness and phase ratio. Thus, a chromatographer will expect a limit in the sample capacity that is set according to the internal diameter for a given phase ratio or film thickness. Small diameter (&lt;0.2 mm internal diameter) columns will offer the advantages of high-efficiency and high-resolution, but at the expense of sample capacity. Low resolution, wide-bore fused silica columns have a much higher sample capacity and are typically employed for performing simple, packed-column separations. Accordingly, the elements of efficiency, speed, capacity, and resolution are factors which must be balanced when optimizing a chromatographic separation.
Accordingly, it would be desirable to employ a capillary column suited for use in the aforementioned modes of chromatography that has the practical advantages of a relatively large internal diameter column (and in particular, its capacity) but also exhibits the high resolution and the rapid analysis obtained by a relatively small internal diameter column.
One approach has been to assemble a plurality of parallel, narrow-bore capillary columns, in which each of the columns presumably have identical properties, in an attempt to eliminate the disadvantage residing in the limited capacity of the narrow-bore capillary column. By using a larger number of such capillary columns coupled in parallel, one would expect to separate large quantities of samples while maintaining the aggregate abilities of each of the capillary columns. However, this proposed "multi-capillary" column device has not been found to be practically useful, due largely in part to the difficulty in providing a column composed of several capillaries coupled in parallel, wherein the individual capillaries exhibit identical behavior. Some examples of the causes of differences that may arise in the performance of individual capillaries are: deformation of the capillaries in different amounts depending on their position in the composite column assembly; temperature gradients over the cross-section of the column (wherein it is difficult to change the temperature of the column without causing radial temperature gradients which subsequently result in different separation rates of the different capillaries); differing stationary phase thicknesses; disparity in column aging, etc. See, for example, U.S. Pat. Nos. 4,424,127 and 4,818,264.